The joining of materials during the manufacture of structures is best achieved through careful cleaning and surface preparation of the substrate. Particularly undesirable surface contaminations include oils that can be found on many metallic components. Such oils can be on a substrate as a consequence of transfer from an upstream process step, such as hydraulic oil from a metal rolling process or slipping oil to aid metallic shaping during pressing and even for corrosion inhibition of ferric metals.
In industrial manufacture the cleaning of the substrates often requires an additional step during the manufacturing process, any such step will by necessity either increase the process complexity, process time, process cost, or all three.
In traditional automotive structure manufacture the problem has been overcome by addition of modifiers to thermosetting adhesives that either adsorb residual oil or use surfactants to perform a chemical substrate de-oiling step as part of the cure process. Either approach uses high curing temperatures of about 180° C. and over an hour in drying ovens.
These relatively high cure temperatures and long durations allow the required joining process to take place. High temperatures provide a low viscosity to enable oil transfer away from the surface. Long duration kinetically allows oil sequestration before the adhesive begins to bond to the metal substrate. This approach achieves the necessary task of curing the adhesive during a required part of the process and necessitates some mechanical fastening until the cure has taken place. Fastening can include local welding or riveting but the use of external clamping is generally considered unworkable.
Epoxy resin based adhesives are widely used for bonding various substrates because of their good bonding strength and versatility. Two-component epoxy resin based adhesive systems in which an epoxy resin in combination with a separate hardener are mixed shortly before use are particularly well-known in the art. However, in such an application the two components (and sometime more than two components) have to be properly measured and thoroughly mixed shortly before use because the epoxy resin component and the hardener component react to form a solid after standing at ambient temperatures for relatively short periods of time, on the order of several minutes to several hours. This poses a problem for large volume industrial applications as the relatively short out-life and the need for accurate mixing are inefficient.
To enable use of advanced composite materials in such industrial structures and to be suitable for use in large volume industrial applications, a bonding adhesive is needed that retains the function of dealing with contaminated substrate surfaces and can cure in less than 5 minutes, ideally in less than 2 minutes. The adhesive must have a long out-life and be supplied in one-component form.
Previous attempts to reduce the time required for the curing reaction by appropriate selection of the epoxy resin or resins used, the amount and nature of the curative and the amount and nature of the catalyst have had limited success in reducing the time required for the curing reaction and have not resulted in an adhesive composition having the desired mechanical properties.
U.S. Pat. No. 4,803,232 describes fast curing by making a two component adhesive. This involves a lot of extra complexity and waste for material storage, mixing and application. The adhesive composition comprises an epoxy resin or polymethylacrylate cured with a combination of an amine-functional butadiene-acrylonitrile rubber, at least one aliphatic or aromatic polyamine and at least one polyamide.
WO 2010/039614 describes a fast curing adhesive but again this relies on a two component format to achieve the balance of storage stability and cure speed. This involves a lot of extra complexity and waste for material storage, mixing and application. The adhesive may be cured at room temperature for at least three hours. The Examples show cure at 180° C. for 30 minutes.
WO 2008/016889 describes a film product with curing at above 150° C. for 10 to 120 minutes. Rubber particles are stably dispersed in the epoxy resin matrix and so remain as separate individual particles.
US2008/0308212 describes an adhesive material including an epoxy resin, an impact modifier and a curing agent. Theoretical exemplary formulations include aliphatic epoxy resins and solid nitrile rubber or solid epoxy adduct in amounts of 15 and 45 weight percent of the adhesive material respectively.